As an LEO, you can use rifles with preban features for duty use only as long the new production/post ban receiver is stamped "Law Enforcement Use Only" (or something similar) on the lower receiver and is registered from the factory as an LE-only weapon.
Just being a sworn LE officer does not exempt you from the 1994 Crime Bill / Assault Weapons ban; you can aquire LE weapons (16" minimum / semi autos only) from most manufactures with simple letterheads signed by both you and the chief. The letterheads would have to state that the firearm being purchased is for duty use only, not for personal use, not for transfer or resale, and the statements are being made under the penalty of perjury. (These are the LE purchase requirements for Olympic Arms, other venders may vary.) Short barreled or select fire models can only be purchased directly by the department and subject to ATF approval.